The present invention relates to a free-electron laser system and a method for generating a free-electron laser beam through the interaction with a laser undulator. The present invention relates to the field of free-electron laser beam generation capable of providing very short-length wave radiation in a spectral range from far ultra-violet to gamma rays. It more particularly relates to a free-electron laser system, comprising means for generating a packet of relativistic electrons capable of propagating in a propagation direction and means for generating an undulator beam capable of interacting with the packet of relativistic electrons. It also relates to a method for generating a free-electron laser beam, including a step of generating a packet of relativistic electrons capable of propagating in a propagation direction, and a step of generating an undulator beam capable of interacting with the packet of relativistic electrons.
In this field, the prior art comprises different types of X-ray lasers amongst which the most currently used are plasma medium X-ray lasers, an example of which is described in U.S. Pat. No. 6,693,989. A system for generating radiation in the X-ray spectral field comprises laser pulse and gaseous state atom packet generators. The laser pulse is directed inside the atom packet such as to produce an atomic excitation, in order to retrieve selected atomic electrons without retrieving all the electrons. A population inversion is thus generated and a non-linear, confined mode radiation propagation area in the X-ray spectral field is established. The density of the atom packet in the gaseous state is controlled in this thus established propagation area such as to ensure that X-ray propagation does not cease.
The invention described in this document makes it possible to obtain a beam of photons the wavelength of which is of 2.9 Angströms. Nevertheless, the system described exhibits the major following drawbacks:                the energy of photons is fixed,        the beam can neither be controlled in divergence, nor in number of photons, nor in pulse duration,        the time synchronization cannot be controlled,        it requires the use of a xenon fluoride gas exciter laser operating at a wavelength of 248 nm, that could be hardly used outside a dedicated research laboratory.        
More generally, the drawback of such laser systems is the inability to reach the spectral X-ray field, but that of XUV rays, in other words, the far ultra-violet X-rays field. Thus, a general issue in this field is to generate a laser beam of which photons may have an energy higher than a typical threshold value in the range of X-rays, for example of 500 eV, i.e., a wavelength of less than 2 nm. Another general issue in this field is the time synchronization of the X laser beam, which can generally be only defined, at best, to within a few hundred femtoseconds.
In order to resolve the abovementioned problems, it has been proposed to implement free electron lasers in Compton regime, based on the use of an electron accelerator coupled with an undulator type structure, able to cause a transverse waviness of electrons. [C. Pellegrini, “Design considerations for a SASE X-ray FEL”, Nucl Inst Meth. A 475 (2001) 1-12]. According to a first alternative of an X-ray free-electron laser, the accelerator causes the electrons to reach energies of 10 GeV and the undulator is a magnetic undulator. This alternative does not however make it possible to attain high X photons energies, ideally clearly higher than 10 keV, while having a significantly reduced cost and bulk.
According to a second alternative of X-ray free-electron laser, the accelerator causes the relativistic electrons to reach energies of around 10 and 100 MeV and the undulator is a laser propagating in the opposite direction to the propagation direction of the relativistic electron packet. This alternative has several major drawbacks. The first is that it requires “delta-gamma/gamma” energy dispersions of the packet electrons which are extremely low (substantially lower than 1%). A second is that it results in very low standardized emittances of the electron packet (substantially lower than 1 mm·mrad). Another difficulty is to achieve a rigorously constant laser illumination in the interaction area (relative illumination variations typically substantially lower than 1%), without a simple possibility of finely controlling the distribution of laser illumination in this area.
Another technical solution to create high flux X-ray beams [Z. Huang and R. D. Ruth “Laser-electron storage ring”. Physical Review Letters, 80(5):976-9, February 1998] would be to use a device comprising a ring for storing relativistic electrons, or a set of one or two energy recovery linear accelerators, coupled with an active or passive laser cavity, in the form of a high-finesse Fabry-Pérot resonator. This compact light source device is also called Fabry-Perot synchrotron. However, this solution has major drawbacks. The first is that the X-radiation is very hardly tunable, i.e., that it is very difficult to modify the energy of the X photons. A second drawback results from the fact that this source does not have its eigen spatial or temporal coherence, outside the spatial coherence induced by the possible free propagation of the beam. Another drawback is the duration of the obtained X pulses, typically around 1 to 10 picoseconds.
Thus, none of the prior art solutions makes it possible to provide laser beams in the far ultraviolet field or X rays which are tunable, have low pulse duration (for example of around 5 to 100 femtoseconds), for a reasonable cost and bulk. Moreover, no prior art known solution makes it possible to provide laser beams in the hard X-ray field (higher than or much higher than 10 keV), or Gamma rays (much higher than 100 keV).
The aim of the present invention is to overcome the deficiencies and drawbacks of the above prior art, by allowing the production of a beam of photons in the spectral fields of the far ultraviolet, X rays and gamma rays, based on a particular interaction between a relativistic packet of electrons and a laser undulator beam from a specific superposition. To this end, the object of the invention is a free-electron laser system such as previously described, wherein the undulator beam results from the superposition, at an interaction area traversed by the propagation direction of the packet of relativistic electrons, of at least two laser beams propagating in directions different from each another, each direction having at least a non zero component in the plane orthogonal to the propagation direction of this packet. The laser beams interfere with each other such as to create a periodic light potential, the minima of which are parallel to the propagation direction of the electrons.
The solution relies on the use of a laser undulator type undulator, wherein the undulator results from the superposition of at least two laser beams. The polarization vectors of these beams make it possible to establish interference fringes such that the energetic potential—called ponderomotive potential—imposed on the relativistic electrons by the laser illumination exhibits oscillations in at least a direction orthogonal to the propagation direction of the relativistic electrons. In these conditions, the relativistic electrons may be trapped in at least a transverse direction by the ponderomotive potential by “Kapitza-Dirac” effect in intense field.
When, in addition, the polarization of the lasers comprises a non-zero common component in a direction orthogonal to the movement direction of the electrons, then a short wavelength radiation beam is formed in the propagation direction of the electrons by:
diffusion of the laser photons on the relativistic electrons with a high spectral shifting through Doppler-Fizeau effect, and
amplification of the first photons diffused by stimulated Raman effect.
According to a particular embodiment of the invention, the beam undulator results from the superposition of two laser beams. In this last case, it may be advantageous, in order to optimize the free-electron laser beam at the output of the system, that the two laser beams be provided such that they propagate in opposite directions and, in addition, that the two laser beams propagate in directions that are perpendicular to the packet propagation direction. Preferably, the packet generating means and the undulator beam generating means are synchronized. Preferably, the undulator beam generating means comprises a laser source the original beam of which is cut such as to form the laser beams.
According to alternative embodiments of the system:
the wavelengths of at least two laser beams are located in the near infrared field (800 nm to 1.05 μm),
the wavelengths of at least two laser beams are located in the mean infrared field (1.1 μm to 10 μm).
According to alternative embodiments, the relativistic electron packet generating means may comprise:
either a LINAC RF (acronym of “radio frequency linear accelerator”,
or a plasma wake laser accelerator,
or an electron storage ring.
According to a particular embodiment, the laser beams propagate in an inhomogeneous wave geometry corresponding to a spatial and temporal patterning such that the locus of the maximum energy of each laser beam exhibits an angular shift with respect to the phase fronts in the interaction area.
Preferably, the laser beams are confined inside an optical resonant cavity. According to alternative embodiments, the system comprises:
phase front controlling means, and/or
spectral phase controlling means.
The present invention also relates to a method for generating a free-electron laser beam. This method comprises a step of generating a packet of relativist electrons capable of propagating in a propagation direction and a step of generating an undulator beam capable of interacting with the relativistic electrons packet. It also comprises, such as to generate the undulator beam, a prior step, at an interaction area traversed by the propagation direction of said packet, of interfering at least two laser beams propagating in different directions and each of which having at least one non-zero component in a plane orthogonal to the propagation direction of said packet, a step of trapping relativistic electrons in the interference fringes of the undulator beam by injection at the input of the interaction area, and a step of amplifying in the undulator of a beam resulting from the diffusion of the lasers on the relativistic electrons.